Multiplex optical communication system

ABSTRACT

A multiplex optical communication system transmitting a multiplex optical signal between terminal equipments each including converters producing optical signals combined to the multiplex optical signal, through repeater equipment including an optical amplifier, under controlling the amplifier so that when a converter is added or removed, a time constant of the amplifier is equal to a time constant of the added or removed converter, or the amplifier produces output under constant output level control before adding or removing the converter and after the amplifier produces final output and constant gain control during the added or removed converter increases or decreases output. The time constant control is performed to the amplifier by making the amplifier repeat start and stop of increasing or decreasing output step by step in accordance with prescribed objective values corresponding to half way output of the optical amplifier.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This is a continuation of U.S. application Ser. No. 09/090,823,filed Jun. 3, 1998, which is a division of U.S. application Ser. No.08/587,390, filed Jan. 17, 1996, now U.S. Pat. No. 5,805,322.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0002] The present invention relates to a multiplex opticalcommunication system for transmitting multiplex optical signals under asignal transmission technology of Wavelength Division Multiplexing (WDM)or Optical Time Division Multiplexing (OTDM).

[0003] In particular, the present invention relates to opticalamplifiers for amplifying power of the multiplex optical signalstransmitted through the system.

[0004]FIG. 1A is a block diagram for illustrating the multiplex opticalcommunication system of the related art, operating under the WDM or OTDMtechnology. The multiplex optical communication system is composed ofoptical signal terminating equipment (TERM EQUIP) (1 and 1′) placed atboth terminals of the OPT-TRANS LINE 3, for transmitting and receivingthe multiplex optical signals, an optical transmission line (OPT-TRANSLINE) (3) made of an optical fiber depicted by a thick line, fortransmitting the multiplex optical signal, and optical amplifierrepeater equipment (OAMP REP EQUIP) (2) placed along the OPT-TRANS LINE3, for amplifying and repeating the multiplex optical signalstransmitting between the TERM EQUIP 1 and 1′.

[0005] The TERM EQUIP 1 includes a transmitting unit (TX-UNIT) (1001)for transmitting a multiplex optical signal to the TERM EQUIP 1′ and areceiving unit (RX-UNIT) (1002) for receiving a multiplex optical signaltransmitted from TERM EQUIP 1′. The same as TERM EQUIP 1, the TERM EQUIP1′ includes TX-UNIT 1001′ and RX-UNIT 1002′. The TERM EQUIP 1 and TERMEQUIP 1′ have the same constitution and function, so that the TERM EQUIP1 will be representatively described hereinafter. The OAMP REP EQUIP 2includes two repeater optical amplifiers (REP OPT-AMPs) (8 and 8′). TheREP OPT-AMP 8 is to amplify the multiplex optical signal transmittedfrom the TX-UNIT 1001 in the TERM EQUIP 1, for repeating the multiplexoptical signal to RX-UNIT 1002′ in the TERM EQUIP 1′, and the REPOPT-AMP8′ is to amplify the multiplex optical signal transmitted from theTX-UNIT 1001′ in the TERM EQUIP 1′, for repeating the multiplex opticalsignal to RX-UNIT 1002 in TERM EQUIP 1. When a distance between TERMEQUIP 1 and 1′ is long, a plurality of the OAMP REP EQUIP 2 are placed.However, one OAMP REP EQUIP 2 is representatively depicted in FIG. 1A.

[0006]FIG. 1B shows a block diagram of the TX-UNIT 1001 of the relatedart. The TX-UNIT 1001 consists of electro-optical signal converter(ELEC-OPT CONV) (4) connected with electrical signal channel lines(ELEC-SIG CHANNEL LINEs) (9) through which a plurality of electricalsignals formed to channels are sent to the ELEC-OPT CONV 4, an opticalsignal combiner (OPT-SIG COMB) (5) connected with the ELEC-OPT CONV 4through optical fibers depicted by thick lines, and a transmitting unitoptical amplifier (TX-UNIT OPT-AMP)(6) connected with the OPT-SIG COMB 5through an optical fiber depicted by a thick line. The ELEC-OPT CONV 4is for converting the electrical signals to optical signals at everychannel. The ELEC-OPT CONV 4 consists of converters 4-1, 4-2, - - - and4-n in correspondence with the ELEC-SIG CHANNEL LINEs 9. When theelectrical signals are fed to the ELEC-OPT CONV 4 through the ELEC-SIGCHANNEL LINEs 9, the converters 4-1, 4-2, - - - and 4-n convert theelectrical signals to optical signals and send the optical signals tothe OPT-SIG COMB 5, respectively. The OPT-SIG COMB 5 is for combiningthe optical signals sent from the ELEC-OPT CONV 4, adopting the WDMtechnology or the OTDM technology, so as to produce a multiplex opticalsignal. The TX-UNITOPT-AMP 6 is for amplifying the power of themultiplex optical signal sent from the OPT-SIG COMB 5. The amplifiedmultiplex optical signal is sent out from the TX-UNIT 1001 to the REPOPT-AMP 8 in OAMP REP EQUIP 2 through the OPT-TRANS LINE 3.

[0007]FIG. 1C shows a block diagram of the RX-UNIT 1002. The RX-UNIT1002 consists of an optical signal branching unit (OPT-SIGBRANCH) (5′)and optical-electro signal converters (OPT-ELEC CONVs) (4′). The OPT-SIGBRANCH 5′ is connected with the OPT-TRANS LINE 3 depicted by a thickline, for optically demultiplexing the received multiplex optical signalto a plurality of received optical signals which are called “receiveddemultiplexed optical signals” hereinafter. The received demultiplexedoptical signals produced at the OPT-SIG BRANCH 5′ are sent to theOPT-ELEC CONV 4′ through optical fibers depicted by thick lines. TheOPT-ELEC CONV 4′ consists of converters 4′-1, 4′-2, - - -, 4′-n at whichthe received demultiplexed optical signals are converted to receivedelectrical signals and sent out from RX-UNIT 1002 to the ELEC-SIGCHANNEL LINEs 9, respectively.

[0008] In FIG. 1A, the OAMP REP EQUIP 2 includes two optical amplifiers(8 and 8′) which will be called REP OPT-AMPs 8 and 8′ hereinafter. TheREP OPT-AMP 8 and 8′ are for amplifying the power of multiplex opticalsignals received from TERM EQUIP 1 and 1′, respectively. By virtue ofthe REP OPT-AMPs 8 and 8′, power loss, caused by the OPT-TRANS LINE 3,of the multiplex optical signals transmitting between TERM EQUIP 1 and1′ are recovered. Therefore, when a length of the OPT-TRANS LINE 3between TERM EQUIP 1 and 1′ is long, a plurality of the OAMP REP EQUIP 2are placed along the OPT-TRANS LINE 3, and the number of the OAMP REPEQUIP 2 is determined by considering both the power loss due to theOPT-TRANS LINE 3 and the power amplification factors of REP OPT-AMPs8and 8′in OAMP REP EQUIP 2, so that the multiplex optical signals can betransmitted between the TERM EQUIP 1 and 1′ in high fidelity and a highsignal to noise ratio (SNR).

[0009] Generally, there are two kinds of optical amplifiers, asemiconductor amplifier and an optical fiber amplifier. The both kindsof optical amplifiers can be applied to the TX-UNIT OPT-AMP 6 in FIG. 1Band the REP OPT-AMPs 8 and 8′ in FIG. 1A. For example, in case theTX-UNIT OPT-AMP 6 is the semiconductor amplifier, the multiplex opticalsignal fed to the TX-UNIT OPT-AMP 6 is amplified by a semiconductordevice operating under DC supply current, and in case the TX-UNITOPT-AMP 6 is the optical fiber amplifier, the multiplex optical signalfed to the TX-UNIT OPT-AMP 6 is amplified in an optically amplifyingtechnology using an induced emission.

[0010] Recently, the optical fiber amplifier is used to the TX-UNITOPT-AMP 6 and the REP OPT-AMP 8 mostly. Because, the optical fiberamplifier has features such as a low Noise Figure, a littlenon-linearity in amplification, a low connection loss with theOPT-TRANSLINE 3, high capability of a power amplification and a highstability against a temperature change. The optical fiber amplifier iscomposed of a rare earth metal-doped optical fiber such as Erbium(Er)-doped optical fiber and a pump light source such as a semiconductorlaser.

[0011] In the multiplex optical communication system of the related art,the output power of the multiplex optical signal from the TX-UNITOPT AMP6 or the REP OPT-AMP 8 is controlled so as to be always constant inlevel under constant output level control performed in the TX-UNIT OPTAMP 6 and the REP OPT-AMP 8 respectively. In case of the REP OPT-AMP 8,by virtue of the constant output level control, the OAMP REP EQUIP 2 canbe placed independently on a length of the OPT-TRANS LINE 3 connectedwith the OAMPREP EQUIP 2. In other words, the power level of eachsection between REP OPT-AMPs or between TERM EQUIP and REP OPT-AMPs isindependent. The change of power level and different of OPT-TRANS LINE 3loss at one section don't affect the power level at next section.

[0012] If a multiplex optical signal includes “n” channels and theTX-UNIT OPT-AMP 6 is required to produce at least output power “P₀” pera channel for obtaining an advisable SNR, the TX-UNIT OPT AMP 6 must bedesigned so as to produce output power of “P₀×n”. In other words, theTX-UNIT OPT AMP 6 initially produces the optical output under theconstant output level control so that the output power of the TX-UNITOPT AMP 6 corresponds to the number of the channels of a multiplexoptical signal to be initially amplified by the TX-UNIT OPT AMP 6.

[0013] From a viewpoint of the operational flexibility of the multiplexoptical communication system, it is desirable that the channels of themultiplex optical signal can be changed easily in response to troubleabout the transmission of the multiplex optical signal and up grade oftraffic capacity. For example, at first some channels which meet demandare used. When more traffic capacity are needed, other channels willbecome used. Usually, the multiplex optical communication systemprovides at least one spare channel in place of a fallen channel. Forexample, when a module of a channel has trouble, another module of thespare channel is used instead of the troubled module. Such previousprovision of the spare channel is effective for increasing theoperational reliability of the multiplex optical communication system.However, when the spare channel is used, there has been a problem of theoutput power in the multiplex optical communication system of therelated art.

[0014] In the multiplex optical communication system of the related art,the constant output level control is performed to optical amplifier soas to keep the total output of the multiplex optical signal constant. Asa result, when the number of the channels decreases by removing a CONVwhich will be called “removed CONV” hereinafter, single output of eachchannel increases. On the contrary, when the number of the channelsincreases by adding a CONV which will be called “added CONV”hereinafter, the single output of each channel decreases.

[0015] When output power of a channel of the multiplex optical signalchanges thus, a problem due to a non-linear effect occurs on the opticalfiber of the OPT-TRANS LINE 3. That is, when the power of a channelexceeds a specific level, a waveform of each channel is distorted by thenon-linear effect on the optical fiber. The non-linear effect isgenerally called a self phase modulation effect. Meanwhile, incontradiction to the above, the power of the optical signal is requiredto be larger than a specific level for maintaining a required SNR at thereceiving unit such as the RX-UNIT 1002 or 1002′.

[0016] In the REP OPT-AMP 8, minimum input power and maximum outputpower are required for performing the reception and the transmission ofthe multiplex optical signal safely.

[0017] When there are a plurality of the REP OPT-AMPs 8 in the multiplexoptical communication system, these minimum input power and maximumoutput power are determined by the reception and amplification abilityof each REP OPT-AMP 8 and the number of the REP OPT-AMPs 8. In each REPOPT-AMP 8, a level difference between the minimum input power and themaximum output power is called a transmission and reception leveldifference. The REP OPT-AMP 8 is designed so that the transmission andreception level difference is larger than a signal loss caused by theOPT-TRANS LINE 3 lying between the OAMP REP EQUIP 2. Furthermore, in thedesign of the REP OPT-AMP 8, a margin of output power of each channel isafforded to insure its level difference caused by increase or decreaseof the number of the channels in the multiplex optical signal. Becauseof allowing the margin thus, a share of the transmission and receptionlevel difference to the optical transmission loss is decreased. In otherwords, a distance between the OAMP REP EQUIP 2 is shortened. Thisresults in increasing the number of the OAMP REP EQUIP 2 uneconomically.

SUMMARY OF THE INVENTION

[0018] Therefore, an object of the present invention is to make themultiplex optical communication system have a large operationalflexibility against the variation of the total number of the channels inthe multiplex optical signal transmitted through the system.

[0019] Another object of the present invention is to increase theoperational fidelity of the multiplex optical communication system.

[0020] Still another object of the present invention is to improvecontradiction occurring in the system that increasing output power isrequired to maintain the required high SNR, however, the output powercannot be increase so high because the self phase modulation effectoccurs in the system.

[0021] Further another object of the present invention is to decreasecosts for constructing and maintaining the repeater equipment bydecreasing the number of the repeater equipment.

[0022] The above objects are achieved by controlling the terminalequipment and the repeater equipment of the system so that outputvariation occurring at the terminal equipment is made equal to theoutput variation occurring at the repeater equipment. Wherein, theconverter is provided in the terminal equipment for converting anelectrical signal to an optical signal. The multiplex optical signal isformed by combining the converted optical signals produced at theconverters in corresponding to a plurality of the electrical signals fedto the terminal equipment. The output variation is caused by changingthe number of operating converters due to adding or removing a converterto or from the operating converters.

[0023] In order to make the optical output variations equal to eachother, either of two ways is operated to the optical amplifier of therepeater equipment while the added converter is increasing output or theremoved converter is decreasing output. One way is “to change the outputpower of the optical amplifier under the same time-constant as theoutput change of the converter” which is called “time constant control”.Another way is “constant gain control” which is control for keeping gainof the optical amplifier constant.

[0024] First of all, “time constant control” will be explained.

[0025] The time constant at changing the target of output level of theoptical amplifier is set as same as the time constant of the addedconverter or the removed converter, then the output power of eachconverter is kept constant even when converter is added or removednewly. Before adding or removing a converter, the terminal and therepeater equipment produce output under “constant output level control”which is for keeping output of the multiplex optical signal producedfrom the terminal and the repeater equipment, constant. When a converteris added or removed, the optical amplifier in the repeater equipmentchanges the target of the prescribed value under the same time constantas the output change of the converter. In order to perform changing thetarget of output level and announce the prescribed value, a monitorcontroller is provided to the terminal equipment for generating anoptical output control signal to be sent to the optical amplifier in therepeater equipment.

[0026] When a converter is added or removed, the monitor controllermonitors the output of the converters and prohibits that an added orremoved converter starts to increase or decrease the output and sendsthe optical output control signal to the optical amplifier in therepeater equipment. After that, the monitor controller permits that theadded or the removed converter starts to raise or decrease the outputpower. The optical amplifier receives the optical output control signaland starts to change the target of output level to the prescribed valuewhich is information on the optical output control signal announced fromthe monitor controller.

[0027] The prescribed value is proportional to the number of theconverters at the terminal equipment.

[0028] Next “constant gain control” will be explained. Before adding orremoving the converter, the optical amplifier of the repeater equipmentproduces output under “constant output level control” which is forkeeping output of the multiplex optical signal produced from theterminal and the repeater equipment, constant.

[0029] When a converter is added or removed, the monitor controllerwhich monitors the number and the output power of converters, prohibitsthat the added or the removed converter starts increasing or decreasingoutput, and sends the optical output control signal to the opticalamplifier. After that, the monitor controller permits the added or theremoved converter starts increasing or decreasing the output power. Uponreceiving the optical output control signal, the optical amplifierchanges “constant output level control” to “constant gain control”.

[0030] After the added or the removed converter finishes increasing ordecreasing the output power, the control of the optical amplifierreturns to the “constant output level control” from “constant gaincontrol”. However, at this time, a constant output level is differentfrom the previous one, because it is a prescribed value determined bythe number of the converters. This is announced to the optical amplifierby the optical output control signal.

[0031] This switching from “constant gain control” to “constant outputlevel control” is performed by the optical output control signal orautomatically, by no signal, after time, which is enough for the addedor the removed converter to finish increasing or decreasing the outputpower, passed.

[0032] When the optical amplifier produces output under the timeconstant control, a delay occurs between the output from the added orremoved converter and the output from the optical amplifier. The delayis shortened by using step-by-step time constant control which isperformed by providing half way objective values in the monitorcontroller so that the added or the removed converter produces outputstep by step under the previously determined rising or falling timeconstant. Every time the added or removed converter starts and stopsincreasing or decreasing output, the optical output control signalincluding the start information is sent from the monitor controller tothe optical amplifier. As a result, the optical amplifier repeatsstarting and stopping the increase or decrease of the outputcorresponding to the objective values. By virtue of the set-by-stepcontrol, the error due to the delay is reduced on an average.

[0033] The optical output control signal is transmitted between theterminal equipment and the repeater equipment through an opticaltransmission line connecting them.

[0034] The improvement described above is performed to the multiplexoptical communication system operating under Wavelength DivisionMultiplexing (WDM) or Time Division Multiplexing (OTDM).

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1A is a block diagram of a multiplex optical communicationsystem of the related art;

[0036]FIG. 1B is a block diagram of a terminal equipment 1001 in themultiplex optical communication system of the related art, in atransmitting mode;

[0037]FIG. 1C is a block diagram of another terminal equipment 1002 inthe multiplex optical communication system of the related art, in areceiving mode;

[0038]FIG. 2 is a block diagram of a multiplex optical communicationsystem of the first embodiment of the present invention;

[0039]FIG. 3 is a block diagram of a transmitting unit in the multiplexoptical communication system of the first embodiment;

[0040]FIG. 4 is a block diagram of an optical amplifier repeaterequipment in the multiplex optical communication system of the firstembodiment, illustrating a connected state with the transmitting unit;

[0041]FIG. 5 is a flow chart for illustrating operation steps in case ofthe first embodiment;

[0042]FIG. 6 is a graph for illustrating an increasing state of opticaloutput from a transmitting unit optical amplifier or a repeater opticalamplifier, in case of the first embodiment;

[0043]FIG. 7 is a graph for illustrating an increasing state of opticaloutput from a transmitting unit optical amplifier or a repeater opticalamplifier, in case of a second embodiment of the present invention;

[0044]FIG. 8 is a block diagram of an electro-optical signal converterfor illustrating the first embodiment;

[0045]FIG. 9 is a block diagram of the transmitting unit opticalamplifier or the repeater optical amplifier, in case of the firstembodiment;

[0046]FIG. 10 is a block diagram of a monitor controller of thetransmitting unit, in case of the first embodiment;

[0047]FIG. 11 is a block diagram of the transmitting unit opticalamplifier or the repeater optical amplifier, in case of the firstembodiment;

[0048]FIG. 12 is a flow chart for illustrating operation steps in caseof a third embodiment of the present invention;

[0049]FIG. 13 is a block diagram of the transmitting unit opticalamplifier or the repeater optical amplifier, in case of the thirdembodiment;

[0050]FIG. 14 is a block diagram of the transmitting unit opticalamplifier or the repeater optical amplifier, in case of the thirdembodiment;

[0051]FIG. 15 is a block diagram of the transmitting unit in case of afourth embodiment of the present invention;

[0052]FIG. 16 is a block diagram of the transmitting unit opticalamplifier or the repeater optical amplifier, in case of the fourthembodiment;

[0053]FIG. 17A is a block diagram of the multiplex optical communicationsystem o f a fifth embodiment of the present invention;

[0054]FIG. 17B is a block diagram of a transmitting unit in themultiplex optical communication system of the fifth embodiment;

[0055]FIG. 17C is a block diagram of the optical amplifier repeaterequipment in the multiplex optical communication system of the fifthembodiment, illustrating a connected state with the transmitting unit;and

[0056]FIG. 18 is wave forms illustrating a multiplex optical signal incase of Optical Time Division Multiplexing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057]FIG. 2 is a block diagram for illustrating a multiplex opticalcommunication system of a first embodiment of the present invention. Inthe first embodiment, the multiplex optical communication systemtransmits signals or data under the OTDM. In FIG. 2, the multiplexoptical communication system principally consists of TERM EQUIP 100 and100′, OAMP REP EQUIP 200 and an OPT-TRANS LINE 3 depicted by a thickline. The TERM EQUIP 100 consists of a TX-UNIT 1011 and a RX-UNIT 1002,the TERM EQUIP 100′ consists of a TX-UNIT1011′ and an RX-UNIT 1002′ andthe OAMP REP EQUIP 200 includes REP OPT-AMPs 81 and 81′. In FIG. 2, thesame reference symbol as in FIG. 1A designates the same unit as in FIG.1A. There is a case where a plurality of OAMP REP EQUIP 200 are placedbetween the TERM EQUIP 100 and 100′ along the OPT-TRANS LINE 3. However,only one OAMP REP EQUIP 200 is representatively depicted in FIG. 2. Amultiplex optical signal transmitted from TX-UNIT 1011 (1011′) isamplified by the REP OPT-AMP 81 (81′) for repeating the multiplexoptical signal to the RX-UNIT 1002′ (1002). The present inventionrelates to optical amplifiers provided in a multiplex opticalcommunication system, so that the present invention relates to TX-UNITs1011 and 1011′ and REP OPT-AMPs 81 and 81′ as shown by blocks diagonallyshaded in FIG. 2. However, the TX-UNITs 1011 and 1011′, REP OPT-AMPs 81and 81′ have the same function and constitution respectively, so that inthe present invention, the TX-UNIT 1011 and REP OPT-AMP 81 will berepresentatively described in reference with FIGS. 3 and 4.

[0058]FIG. 3 is a block diagram for illustrating the TX-UNIT 1011 andFIG. 4 is a block diagram for illustrating a relationship between theTX-UNIT 1011 and the REP OPT-AMP 81. In FIGS. 3 and 4, the samereference symbol as in FIG. 1B designates the same unit as in FIG. 1B.

[0059] In FIG. 3, the TX-UNIT 1011 consists of an ELEC-OPT CONV 4, anOPT-SIG COMB 5 optically connected with the ELEC-OPT CONV 4throughoptical fibers depicted by thick lines, a TX-UNIT OPT-AMP 6 opticallyconnected with the OPT-SIG COMB 5 by an optical fiber depicted by athick line and a monitor controller (MON CONT) (7) electricallyconnected with the ELEC-OPT CONV 4, the TX-UNITOPT-AMP 6 and the REPOPT-AMP 81, respectively. The ELEC-OPT CONV 4 consists of convertermodules (CONVs) (4-1, 4-2, - - - and 4-n) each including anelectro-optically converting circuit (CONV-CIRCUIT) (10), an opticaloutput monitor (OPT-OUT MON) (11) and an optical output controller(OPT-OUT CONT) (12).

[0060] Electrical signals formed to channels are fed to theCONV-CIRCUITs in the CONVs 4-1, 4-2, - - - and 4-n throughELEC-SIGCHANNEL LINEs 9-1, 9-2, - - - and 9-n, respectively. In case ofthe WDM, the CONV-CIRCUITs 10 produce optical signals having a differentfrequency or wavelength each other. The OPT-OUT MONs 11 monitor theoptical output from the CONV-CIRCUITs 10 respectively, producingmonitored optical output signals. The monitored optical output signalsare sent to the MON CONT 7 and to the OPT-OUT CONTs 12 in the CONVs 4-1,4-2, - - - , 4-n.

[0061] The MON CONT 7 consists of a monitor processor (MON-PROC) (13), acontrol signal processor (CONT-PROC) (14) and a control signaltransmitter (CONT-SIG TX) (15). The monitored optical output signalssent from the OPT-OUT MONs 11 are collected at the MON-PROC 13,producing a collected monitored signal. The collected monitored signalis sent to the CONT-PROC 14 at which an optical output control signal isproduced and sent to both the OPT-OUT CONTs 12 and the CONT-SIG TX 15.The CONT-SIG TX 15 is for transmitting the optical output control signalto the TX-UNIT OPT-AMP 6 in TX-UNIT 1011 of the TERM EQUIP 100 and tothe REP OUT-AMP 81 in the OAMP REP EQUIP 200. There is a case where theTX-UNIT 1011 includes no TX-UNIT OPT-AMP 6. In this case, the opticaloutput control signal is sent only to the REP OPT-AMP 81. If there are aplurality of the OAMP REP EQUIP 200, the optical output control signalis sent to them through the REP OPT-AMP 81 in the OAMP REP EQUIP 200.

[0062] In each CONV, upon receiving the monitor output from the OPT-OUTMON 11 and the optical output control signal from the CONT-PROC 14, theOPT-OUT CONT 12 performs stabilization and increase or decrease controlof the optical output from the CONV-CIRCUIT 10 by setting a rising or afalling time constant around the CONV-CIRCUIT 10.

[0063] In the MON CONT 7, when the MON-PROC 14 collects the opticaloutput monitored signals from the OPT-OUT MONs 11 in the CONVs 4-1 to4-n, the CONT-PROC 14 investigates how the optical signal is producedfrom the CONV-CIRCUIT 10 in each CONV. After the investigation, theCONT-PROC 14 produces the optical output control signal to start or stopincreasing or decreasing the optical output of the CONV-CIRCUIT 10 in aCONV required to be added or removed. The CONT-PROC 14 has anotherfunction for producing the start-stop control signal when a start signal“st” or a reset signal “rt” is given through an interface, not depictedin FIG. 3, provided in the TERM EQUIP 100. The optical output controlsignal produced at the CONT-PROC 14 is sent to the OPT-OUT CONT 12 ineach CONV, the TX-UNIT OPT-AMP 6 in the TX-UNIT 1011 and the REP OPT-AMP81 in the OAMP REP EQUIP 200 through the CONT-SIG TX 15.

[0064] In FIG. 3, if the number of the CONVs 4-1, 4-2, - - - and 4-n is“a” (n=a) and an optical output per a single channel, which will becalled a “single channel optical output” hereinafter, from each CONV ishypothesized to be equally “1” in a steady state, and when a CONV4-(a+1) is newly added to the ELECT-OPT CONV 4, optical output of allchannels, which will be called “all channel optical output” hereinafter,of the TX-UNIT OPT-AMP 6 becomes as

a+1−exp(−t/τ1).  (1)

[0065] In the expression (1), “t” is a lapse of time measured from timeto start operation of the CONV 4-(a+1) and “τ1” is a time constantrequired to make the CONV 4-(a+1) produce its single channel opticaloutput completely.

[0066] If a single channel optical output “p” is produced per eachchannel from the TX-UNIT OPT-AMP 6 or the REP OPT-AMP 8 and the allchannel optical output is increased from “p×a” to “p×(a+1)” after a timeconstant τ2 and when τ1 and τ2 are equal to τ1(τ1=τ2=τ), the all channeloptical output from the TX-UNIT OPT-AMP 6 or the REP OPT-AMP 8 can beexpressed by:

p×a+{(p×(a+1)−(p×a))×{1−exp(−t/τ)}=p×a+p×{1−exp(−t/τ)}.  (2)

[0067] At this time, during the transitional state, the single channeloptical output from the TX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 is givenas:

p×a+p×{1−exp(−t/τ)}×{1−exp(−t/τ)}×{1/(a+1−exp(−t/τ)}=p.  (3)

[0068] The expression (3) signifies that when a CONV 4-(a+1) is added tothe ELEC-OPT CONV 4 and starts to operate, a single channel opticaloutput from the TX-UNIT OPT-AMP 6 (and the REP OPT-AMP 81) can be keptto “p”, by making the time constant τ2 equal to the time constant τ1. Byvirtue of changing output power of the TX-UNIT OPT-AMP 6 (and the REPOPT-AMP 81) as controlling the time constant τ2 so as to be nearly equalto τ1, the problem of increasing the waveform distortion occurring inthe multiplex optical signal due to the non-linear effect of theOPT-TRANS LINE 3 can be solved.

[0069] In opposition to the above, when one of the CONVs 4-1 to 4-a isremoved by stopping operation, a single channel optical output from theTX-UNIT OPT-AMP 6 (and the REP OPT-AMP 81) can be always kept to “p”, bycontrolling a decrease rate of the all channel optical output from theTX-UNIT OPT-AMP 6 (and the REP OPT-AMP 81) so that a time constant (τ4)required for decreasing the level of the all channel optical output fromthe TX-UNIT OPT-AMP 6 (and the REP OPT-AMP 81) to a value “p×(a−1)” isthe same as a time constant (τ3) required for extinguishing the singlechannel optical output of the removed CONV to a zero level. By virtue ofcontrolling thus, the problem of increasing the waveform distortionoccurring in the multiplex optical signal due to the non-linear effectof the OPT-TRANS LINE 3 can be solved.

[0070] The time constant such as τ1 or τ2 in the above description willbe called “rising time constant” and the time constant such as τ3 or τ4will be called “falling time constant” hereinafter.

[0071] In FIG. 3, the MON CONT 7 always monitors the optical output fromthe CONVs 4-1 to 4-n at the MON-PROC 13, and when a level of the opticaloutput from a CONV is varied, the variation is processed at the MON-PROC13 and the processed result is sent to the CONT-PROC 14 at which theoptical output control signal is produced for controlling the allchannel optical output from the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81in correspondence with the level variation of the optical output of theCONVs 4-1 to 4-n. The optical output control signal is transmitted fromthe CONT-SIG TX 15 to the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81respectively.

[0072] The first embodiment of the present invention is based oncontrolling the time constant τ2 so as to be equal to τ1 as described inreference with the expression (3). The “control” will be called “timeconstant control” and system on the “time constant control” will becalled “time constant control system” hereinafter. FIG. 5 is a flowchart for illustrating the operation steps in case of the firstembodiment. In the first embodiment, the ELECT-OPT CONV 4, the MON CONT7 and the TX-UNIT OPT-AMP 5 in the TX-UNIT 1011 of the TERM EQUIP 100and the REP OPT-AMP 81 in OAMP REP EQUIP 200 operate in accordance withsteps A1 to A9 in FIG. 5. In the description of the steps A1 to A9, theadded CONV will be used for the CONV 4-(a+1) and a removed CONV will beused for a CONV removed from the ELEC-OPT CONV 4 hereinafter.

[0073] Whether CONV is added or removed is determined by a maintenanceworker (step A1, CONV ADDED OR REMOVED?). In case a CONV is added, theadded information is sent to the MON CONT 7 by an installation monitorunit which will be described later, a higher rank apparatus not depictedin FIGS. 3 and 5, or the maintenance worker (step A2, SEND ADD-INF).Upon receiving the added information, the CONT-PROC 14 in the MON CONT 7issues a command to raise optical output to the added CONV, and theCONT-SIG TX 15 in the MON CONT 7 issues a command to raise opticaloutput to the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81 respectively(step A3, RAISE OUTPUT OF ADD-CONV AND OPT-AMPs). The added CONVincreases the single channel optical output to a prescribed value by thecommand issued from the CONT-PROC 14 passing through the OPT-OUT CONT 12in the added CONV, and the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81increase the optical output at a rate determined by the time constant τ2which is previously set up in equal to the rising time constant τ1 ofthe added CONV (step A4, OPT-AMPs INCREASE OUTPUT WITH τ2 EQUAL TO τ1 OFADD-CONV). When the single channel optical output of the added CONVbecomes the prescribed value and the optical output from the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81 become respectively a value equal tothe product of the single channel optical output, the added CONV, theTX-UNIT OPT-AMP 6 and the REP OPT-AMP 81 maintain the product valuerespectively. For example, the added CONV, the TX-UNIT OPT-AMP 6 and theREP OPT-AMP 81 maintain the product value “p×(a+1)” respectively in case“p” is the power of the single channel output and “a+1” is the totalnumber of the operating CONVs (step A5, OPT-AMPs OUTPUT p×(a+1)STEADILY).

[0074] In case a CONV is removed from the ELEC-OPT CONV 4 because oftrouble or maintenance, the remove information is sent to the MON CONT 7from a higher rank apparatus not depicted in FIGS. 3 and 5, or themaintenance worker (step A6, SEND REMOV-INF). Upon receiving theremoving information, the MON CONT 7 sends a command to decrease singlechannel optical output to there moved CONV and a command to decrease theoptical output to the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81respectively (step A7, DECREASE OUTPUT OF REMOVE-CONV AND OPT-AMPs).Upon receiving the decreasing command, the removed CONV decreases thesingle channel optical output toward zero and the TX-UNIT OPT-AMP 6 andthe REP OPT-AMP 81 decrease the optical output at a rate determined by atime constant (τ4) which is previously set up in equal to the fallingtime constant (τ3) of there moved CONV (step A8, OPT-AMP DECREASE OUTPUTWITH τ4 EQUAL TO τ3 OF REMOVE-CONV). Then, when the TX-UNITOPT-AMP 6 andthe REP OPT-AMP 81 become respectively a value being the product of thesingle channel optical output, for example “p”, and the number, forexample “a−1”, of operating CONVs, the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81 maintain the product value “p×(a−1)” respectively and theremoved CONV is completely stopped in operation (step A9, OPT-AMPsPRODUCEOUTPUT p×(a−1) STEADILY).

[0075]FIG. 6 is a graph for illustrating an increasing state of theoptical output from the TX-UNIT OPT-AMP 6 (or the REP OPT-AMP 81). InFIG. 6, a y-axis indicates the optical output of the TX-UNIT OPT-AMP 6and an x-axis indicates time. When the TX-UNIT OPT-AMP 6produces output“p” for a single optical signal and the operating number of the CONVs is“a”, the optical output of the TX-UNITOPT-AMP 6 becomes p×a finallywhich will be called P₁(p×a=P₁), and when a CONV is added to theELECT-OPT CONV 4 and the added CONV starts to operate at time t1, theoptical output of the TX-UNIT OPT-AMP 6 becomes p×(a+1) finally whichwill be called P₂(p×(a+1)=P₂). In FIG. 6, a solid line indicates anideal increasing state of the optical output of the TX-UNIT OPT-AMP 6from P₁ to P₂. A rate of increasing is determined by the rising timeconstant τ2 of the TX-UNIT OPT-AMP 6 which is controlled so as to beequal to the rising time constant τ1 of the added CONV. By virtue ofmaking the rising time constant τ2 equal to the rising time constant τ1,the output power of the single optical signal from the TX-UNIT OPT-AMP 6can be avoided varying. This results in preventing the waveformdistortion due to the non-linearity effect of the OPT-TRANS LINE 3 andthe SNR degradation from occurring in the multiplex opticalcommunication system.

[0076] However, there is a case where a time delay occurs between thetime required to raise the optical output of the added CONV and a timerequired to increase the optical output of the TX-UNIT OPT-AMP 6. Adotted line in FIG. 6 indicates an actual state of increasing theoptical output of the TX-UNIT OPT-AMP 6 from P1 to P2 in case the timedelay occurs. FIG. 6 shows that though the added CONV rises the outputpower during time t1 to t2, the TX-UNIT OPT-AMP 6 increases the opticaloutput during time t1+Δt to t2+Δt having a delay time Δt. That is,because of the delay time Δt, an error ΔP₁ of output power appearsbetween the actual state of power increasing shown by the dotted lineand the ideal state of power increasing shown by the solid line.Incidentally, the time interval between tl and t2 is several ms toseveral second and Δt is several % to approximately 25% of the timeinterval.

[0077] The error ΔP₁ of output power can be decreased by improving thetime constant control so that the output power of the added CONV risesin a step-by-step manner as shown in FIG. 7. FIG. 7 illustrates thestep-by-step manner for increasing the output power of the added CONVunder the improved time constant control. In FIG. 7, symbols P₁₁, P₁₂and P₁₃ indicate halfway output from the TX-UNIT OPT-AMP 6 to be reachedat halfway times in the time interval between t1 and t3. The added CONVproduces optical output having levels corresponding to halfway objectivevalues, a first objective value, a second objective value and a thirdobjective value, in correspondence with P₁₁, P₁₂ and P₁₃ produced at thehalfway times respectively.

[0078] Before the added CONV is added, the output power of the TX-UNITOPT-AMP 6 is P₁ steadily. When the added CONV is added and it starts toraise optical output at time t1, the TX-UNIT OPT-AMP 6 starts toincrease output power under the time constant control, at time t1 ′delayed an amount of Δt from time t1. When the optical output of theadded CONV increases as depicted by a thick line in FIG. 7 and reaches alevel corresponding to the first objective value, the output of theadded CONV is stopped to be raised until the TX-UNIT OPT-AMP 6 producesthe optical output P₁₁. After the optical output of the TX-UNIT OPT-AMP6 increases as depicted by a dotted line and reaches P₁₁, the added CONVstarts to rise the optical output at time t1″ and rises the opticaloutput until the optical output reaches a level corresponding to thesecond objective value. After delaying, At from time t1″, theTX-UNITOPT-AMP 6 starts to rise the optical output and the opticaloutput increases until becoming P₁₂. These steps are repeated step bystep, increasing the output power of the TX-UNIT OPT-AMP 6 toward P₂.

[0079] When the output of the TX-UNIT OPT-AMP 6 reaches P₂ at time t3,the step-by-step increasing operation is ended.

[0080] In FIG. 7, under the step-by-step manner, an error ΔP₂ of theoutput power of the TX-UNIT OPT-AMP 6 appears between the actualincreasing state shown by the dotted line and the ideal increasing stateas shown by the solid line. However, since the error ΔP₂ appearsintermittently as shown in FIG. 7, an average error obtained by summingindividual error ΔP₂ from t1 to t2 becomes small in comparison with anaverage error the error ΔP₁ in between time t1 and time t2+Δt in FIG. 6.The details of the multiplex optical communication system operatingunder the time constant control performed in the step-by-step mannerwill be described later as a second embodiment of the present invention.

[0081]FIG. 8 is a block diagram for illustrating one of the CONVs 4-1 to4-n in case of the first embodiment. The one of the CONVs 4-1 to 4-nwill be called simply “CONV” hereinafter. In FIG. 8, the same referencenumeral as in FIG. 3 designates the same unit as in FIG. 3. The CONVprincipally consists of the CONV-CIRCUIT 10, the OPT-OUT MON 11 and theOPT-OUT CONT 12. The CONV-CIRCUIT 10 consists of a semiconductor laserdiode (LD) (21) for emitting laser light, an optical modulator (OPT-MOD)(22) for producing a single channel optical signal by intentionallymodulating laser light emitted from the LD 21, and a driving circuit(DRIVE) (3) for driving the OPT-MOD 22 upon receiving an electricalsignal or data to be transmitted, through the ELEC-SIG CHANNEL LINE 9.The OPT-OUTMON 11 consists of an operation monitoring circuit (OPT-MONCIRCUIT) (24) for monitoring the output power of the laser uponreceiving a part of laser light emitted from the LD 21, an indicator(25) for indicating a monitored result obtained by receiving a part ofmonitored output from the OPT-MON CIRCUIT 24 and an installation monitorunit (INSTALL-MON) (26) for monitoring that the CONV is installed in theELECT-OPT CONV 4. The OPT-OUT CONT 12 consists of a low pass filter(LPF) (27), a bias generating circuit (BIASGEN) (28) for generating abias voltage applied to the LD 21 and a switch (SWITCH) (29) forswitching a source voltage for the BIASGEN 28. The rising and fallingtime constant can be previously varied and set for the CONV by adjustingthe LPF 27. Terminals “a, b, c and d” are for connecting the CONV withthe MON CONT 7, terminal “e” is a power source terminal and terminal “f”is a terminal connected with the ELECT-SIG CHANNEL LINE 9 for receivingthe electrical signal or data to be transmitted.

[0082] When the SWITCH 29 is ON by the optical output control signalsent from the MON CONT 7 through terminal “d”, DC power is applied tothe BIAS GEN 28 through terminal “e” and a bias voltage generated at theBIAS GEN 28 is supplied to the LD 21 through the LPF27. The opticaloutput power from the LD 21 is monitored by the OPT-MON CIRCUIT 24, sothat the LD 21 transmits optical power finally having the objectivevalue. In this case, the optical power transmitted from the LD 21 isgradually raised in accordance with the bias voltage gradually raised.In other words, when the LD 21 is started to transmit the optical power,the optical power rises gradually at a rate determined by a timeconstant (τ1) corresponding to a rising rate of the bias voltagegenerated at the BIAS GEN 28.

[0083] The OPT-MON CIRCUIT 24 monitors the optical output from the LD21. When an accident occurs in the CONV, the OPT-MONCIRCUIT 24 issues acommand to stop the operation of the CONV through terminal “c” and atthe same time, the OPT-MON CIRCUIT24 controls the indicator 25 so as tomake the INDICATOR 25 indicate an accident occurrence. The indicator 25has a function of indicating a state of “normally operating”, “varyingoptical output”, “accident occurs” or “stopping operations”. When manyCONVs are installed in a rack, the indicator 25 is very helpful todistinct the operation state of the CONVs and very useful to performquick exchange of a CONV when an accident occurs in the CONV.

[0084] The INSTALL-MON 26 informs to the MON CONT 7 that the CONV isinstalled in the ELEC-OPT CONV 4 by using a jumper wire provided on aback-board of the INSTALL-MON 26. When the CONV is installed in theELEC-OPT CONV 4, terminals “a” and “b” is electrically connected by thejumper wire. By virtue of connecting “a” and “b”, the installation ofthe CONV to the ELEC-OPT CONV4 is informed to the MON CONT 7.

[0085]FIG. 9 is a block diagram for illustrating the TX-UNIT OPT-AMP 6shown in FIG. 3 or the REP OPT-AMP 81 shown in FIG. 3, used in the firstembodiment. The TX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 consists of anoptical amplifier unit (OPT-AMP UNIT) (31), an optical output monitoringunit (OUTPUT MON) (32), a gain controlling unit (GAIN CONT) (33), anoutput constantly control unit (OUTPUT CONT) (34), an output levelsetting unit (LEVEL SET) (35), a control unit (CONT UNIT) (36) and acontrol signal receiving unit (CONT-SIG RX) (37).

[0086] The OPT-AMP UNIT 31 is an optical amplifier composed of anErbium-doped fiber amplifier and a pump light semiconductor laser. TheOUTPUT CONT 34 controls the OPT-AMP UNIT 31 through the GAIN CONT 33 sothat the optical output of the OPT-AMP UNIT 31 becomes a prescribedconstant level by making a comparison between a level of optical outputdetected by the OUTPUT MON 32 and an output level set at the LEVEL SET35.

[0087] Upon receiving the optical output control signal from the MONCONT 7 (refer FIG. 3), the CONT-SIG RX 37 sends the optical outputcontrol signal to the CONT UNIT 36 and other REP OPT-AMPs 81. Uponreceiving the optical output control signal, the CONT UNIT 36 controlsthe LEVEL SET 35 so that the LEVEL SET 35 produces a setting level ofthe optical output from the OPT-AMP UNIT 31 at a gradually increasing ordecreasing rate. When the CONV is added, the setting level is graduallyincreased in accordance with the rising time constant of the added CONV,and when the CONV is removed, the setting level is gradually decreasedin accordance with the falling time constant of the removed CONV. Thesetting level produced at the LEVEL SET 35 is sent to the OPT-AMP UNIT31 through the OUTPUT CONT 34 and the GAIN CONT 33 in order to increaseor decrease the level of the optical output from the OPT-AMPUNIT 31. Theoptical output level of the OPT-AMP UNIT 31 is monitored by the OUTPUTMON 32 and the monitored output from the OUTPUT MON 32 is sent to theOUTPUT CONT 34. When the monitor output reaches a level corresponding tothe number of the CONVs (refer FIG. 3), the OUTPUT CONT 34 controls sothat the level is always kept constant. Wherein, the number is increasedwhen CONV is added and the number is decreased when CONV is removed. TheTX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 will be detailed later inreference with FIG. 11.

[0088]FIG. 10 is a block diagram for illustrating the MON CONT 7 (referFIG. 3) used in the first embodiment of the present invention. In FIG.10, the same reference numeral as in FIG. 3 designates the same unit asin FIG. 3 and the same reference symbol as in FIG. 8 designates the samepart as in FIG. 8. As shown in FIGS. 3 or 10, the MON CONT 7 principallyconsists of the MON-PROC 13, the CONT-PROC 14 and the CONT-SIG TX 15. InFIG. 10, the MON-PROC 13 and the CONT-PROC 14 are shown in adot-dash-line box respectively and the CONT-SIG TX 15 is shown in asolid line box.

[0089] The MON-PROC 13 consists of installation detecting circuits(INST-DET CIRCUIT) (41-1, 41-2, - - - and 41-n) and monitor controlcircuits (MON-CONT CIRCUITs) (45-1, 45-2, - - - and 45-n) incorresponding to the CONVs 4-1, 4-2, - - - and 4-n respectively. TheINST-DET CIRCUITs 41-1, 41-2, - - - and 41-n are for detecting whetherthe CONVs 4-1, 4-2, - - - and 4-n are installed in the TX-UNIT 1011respectively. Each of the INST-DET CIRCUITs 41-1, 41-2, - - - and 41-nconsists of a resister (42), a standard voltage source (43) and acomparator (COMP) (44).

[0090] When one of the CONVs 4-1, 4-2, - - - and 4-n is not installed inthe ELECT-OPT CONV 4, a terminal “a” and “b” is not electricallyconnected with the jumper wire in the INSTALL-MON 26 shown in FIG. 8.Therefore potential at the terminal “b” becomes higher than earthpotential at the terminal “a”. Upon detecting a difference of thepotential between the terminals “a” and “b”, the COMP 44detects that theremoved CONV is not installed in the ELEC-OPT CONV 4. When, for example,a CONV is newly installed in the TX-UNIT 101 1, terminal “a” and “b” areconnected with the jumper wire in the INSTALL-MON 26, so that potentialat the terminal “b” becomes equal to the earth potential at the terminal“a”. Upon detecting the equal earth potential at the terminal “a” and“b”, the COMP 44 detects that the CONV is installed in the ELEC-OPT CONV4. Upon receiving the potential difference between the terminals “a” and“b” from the COMP 44 and the command to stop the operation of theremoved CONV or to start the operation of the added CONV throughterminal “c”, the MON-CONT CIRCUIT 45-1 sends a stop operation signal toa latch circuit 46-x or a start operation signal to a latch circuit46-(a+1) in the CONT-PROC 14, respectively. The latch circuits areprovided in the CONT-PROC 14 which will be described below.

[0091] The CONT-PROC 14 consists of latch circuits (LATCHes) (46-1,46-2, - - - and 46-n), a trigger generator (TRIG GEN) (47), a counter(COUNT) (48) and a control signal generator (CONT-SIG GEN) (49).

[0092] The COUNT 48 holds the total number of the operating CONVs andproduces a count up signal and performs up-counting or down-counting inaccordance with the stop operation signal or the start operation signalsent from relevant one of the MON-CONTCIRCUITs 45-1, 4-2, - - - and45-n, and when the counting is completed, the COUNT 48 controls the TRIGGEN 47 so that the TRIGGEN 47 adds a trigger signal to the LATCHes 46-1,4-2, - - - and 46-n and to the CONT-SIG GEN 49. Upon receiving thetrigger signal, the relevant one of the LATCHes 46-1, 4-2, - - - and46-n latches the stop operation signal or the start operation signalsent from the relevant one of MON-CONT CIRCUITs 45-1, 45-2, - - - and45-n. For example, when the CONV 4-1 is installed and starts to operate,the start operation signal produced at the MON-CONT 45-1 is sent toterminal “d” through the LATCH 46-1 opened up by a trigger signal sentfrom the TRIG GEN 47. The CONT-SIG GEN 49 is also controlled by anoutput signal from the COUNT 48 and the trigger signal from the TRIG GEN47, so that the CONT-SIG GEN 49 produces the optical output controlsignal for starting or stopping increasing or decreasing output of theTX-UNIT OPT-AMP 6 and REP OPT-AMP 81. Upon receiving the optical outputcontrol signal from the CONT-SIG GEN 49, the CONT-SIG TX 15 transmitsthe optical output control signal to the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81 respectively.

[0093]FIG. 11 is a block diagram for illustrating a detail of theTX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 used in the first embodiment ofthe present invention. In FIG. 11, the same reference numeral as in FIG.9 designates the same unit as in FIG. 9. As described in reference withFIG. 9, the TX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 principally consistsof the OPT-AMP UNIT 31, the OUTPUTMON 32, the GAIN CONT 33, the OUTPUTCONT 34, the LEVEL SET 35, the CONT UNIT 36, the CONT-SIG RX 37 and acontrol signal regenerator (CONT-SIG REGENE) (58). The GAIN CONT 33consists of an optical combiner (51) and a semiconductor laser (52), theOUTPUT MON 32 consists of an optical branch (53) and a photo diode (54),the OUTPUT CONT 34 consists of a transistor amplifier (55) and anoperational amplifier (56) and the LEVEL SET 35 includes a switchingcircuit (SW) (57). The OPT-AMP UNIT 31 is composed of an Erbium-dopedoptical fiber. FIG. 11 shows the Erbium-doped optical fiber in a casewhere forward pumping is performed by a semiconductor laser (52).Instead of the forward pumping, backward pumping or both forward andbackward pumping can be used.

[0094] Moreover, an isolator, not depicted in FIG. 11, for interruptinga turn back light of the OPT-AMP UNIT 31 can be added to the OPT-AMPUNIT 31. Usually, a wavelength in a band of 1.48 μm or 0.98 μm can beused for an optical signal and wavelength in a band of 1.55 μm can beused for a pumping light emitted from the laser 52.

[0095] The operational amplifier 56 compares one of reference values“rf1, rf2, - - - or rfm” switched by the SW 57 with the optical outputdetected at the OUTPUT MON 32, producing comparison output. Uponreceiving the comparison output from the operational amplifier 56, thetransistor amplifier 55 controls the laser 52 so that the pumping lightemitted from the laser 52 makes the OPT-AMP UNIT 3 l produce the opticaloutput at a constant level.

[0096] The optical output control signal transmitted from the CONT-SIGTX IS (refer FIG. 3) is sent to the CONT UNIT 36 through the CONT-SIG RX37. The CONT UNIT 36 controls the SW 57 so that the SW 57 switcheseither one of the reference values. By virtue of the switched referencevalue, the optical output of the OPT-AMP UNIT 31 becomes a levelcorresponding to the total number of actually operating CONVs, obtainedafter increasing or decreasing the CONV. The reference values “rf1,rf2, - - - or rfm” are minutely provided and switched by the SW 57, forincreasing or decreasing the optical output of the OPT-AMP UNIT 31finally in corresponding to the rising time constant of the added CONVor the falling time constant of the removed CONV, respectively. The SW57 is controlled by the CONT UNIT 36 so that the switching is closelyperformed as time passes in the rising or falling process of the outputpower of the added or removed CONV. We design that the time constant ofthe OUTPUT CONT 34 equal to the rising time constant of the added CONVor the falling time constant of the removed CONV.

[0097] The step-by-step manner as shown in FIG. 7 is also performedunder the time constant control performed by the MON CONT 7. In thiscase, one of the MON-CONT CIRCUITs 45-1 to 45-n and the CONT-SIG GEN 49send the optical output control signal to the OPT-MON CIRCUIT 24 in theCONV, the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81, under condition ofthe rising or falling time constant, the halfway objective values andprescribed times to attain the halfway objective value.

[0098] Upon setting the halfway objective values, the prescribed timesand the rising or falling time constant at the CONV such as the CONV4-1,4-2, - - - or 4-n and the OPT-AMP such as the TX-UNIT OPT-AMP 6 and theREP OPT-AMP 81, the optical output from the TX-UNIT OPT-AMP 6 and theREP OPT-AMP 81 is controlled so as to be increased or decreased in thestep-by-step manner.

[0099] A third embodiment of the present invention is based on changinga controlling manner of the optical output from the optical amplifierused in the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81. System forchanging the control manner thus will be called “control manner changesystem” hereinafter. By virtue of the control manner change system, theoptical amplifier is controlled as follows:

[0100] usually the optical amplifier is controlled so as to produceoptical output at a constant level under the constant output levelcontrol, as done in the prior art;

[0101] when a CONV is added to or removed from the ELEC-OPT CONV 4, theconstant output level control is changed to constant gain control, sothat the optical amplifier is controlled so as to produce the opticaloutput under the constant gain control; and

[0102] when the optical output of the added CONV is completely raised orthe optical output of the removed CONV is completely decreased, theconstant gain control is changed back to the usual constant output levelcontrol, so that the optical amplifier is controlled so as to producethe optical output under the constant output level control.

[0103]FIG. 12 is a flowchart for illustrating operation steps of thecontrol manner change system used in the third embodiment. The OPT-AMPusually operates under the constant output level control system. Upondetermining whether the CONV is added to or removed from the ELEC-OPTCONV 4 (step B1, CONV ADD or REMOVE?), the OPT-OUT MON 11 in the CONV(see FIG. 8) or a maintenance worker informs to the MON CONT 7 that aCONV is added (step B2, SEND ADD-INF), in case the CONV is added. Thenthe MON CONT 7 issues a command to switch the constant output levelcontrol to constant gain control to the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81 and another command to raise optical output to the added CONV(step B3, SWITCH CONST-LEVEL-CONT TOCONST-GAIN-CONT, AND RAISE OUTPUT OFADD-CONV).

[0104] When the optical output of the added CONV reaches the prescribedvalue, the MON CONT 7 issues a command to switch the constant gaincontrol back to the constant level control to the TX-UNIT OPT-AMP 6 andthe REP OPT-AMP 81 (step B4, SWITCH BACK TOCONST-LEVEL-CONT). As aresult, the total optical output of the ELEC-OPT CONV 4, the TX-UNITOPT-AMP 6 and the REPOPT-AMP 81 is set to a prescribed valuecorresponding to the number of the operating CONVs, respectively (stepB5, OUTPUT OFELEC-OPT CONV, OPT-AMPs 6 and 81 ARE SET TO PRESCRIBEDVALUE).

[0105] When a CONV is removed from the ELEC-OPT CONV 4 because of, forexample, trouble or maintenance, the remove information is sent to theMON CONT 7 from a higher rank apparatus or by a maintenance worker (stepB6, SEND REMOVE-INF). The MON CONT 7 issues a command to switch theconstant output level control to the constant gain control to both theTX-UNIT OPT-AMP 6 and the REP OPT-AMP 81 and a command to decrease theoptical output to the removed CONV (B7, SWITCH CONST-LEVEL-CONTTOCONST-GAIN-CONT AND DECREASE OUTPUT OF REMOVE-CONV). After the opticaloutput of the removed CONV falls to a prescribed value, the MONT CONV 7issues a command to switch the constant gain control back to theconstant output level control to the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81. Or after a prescribed time passes, the control of theTX-UNIT OPT-AMP6 and the REP OPT-AMP 81 may switch automatically theconstant gain control back to the constant output level control (stepB8, SWITCH BACK TO CONST-LEVEL-CONT). As a result of step B8, theoperation of the removed CONV is completely stopped and the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81 produce the optical output of aprescribed value corresponding to the number of the operating CONVsexcluding the removed CONV, under the constant output level control(step B9, REMOVE-CONV STOP ANDOPT-AMPs 6 AND 81 PRODUCE OUTPUTCORRESPONDING TO THE NUMBER OF OPERATING CONVs UNDER CONST-LEVEL-CONT).

[0106]FIG. 13 is a block diagram of an optical amplifier used to theTX-UNIT OPE-AMP 6 and the REP OPT-AMP 81 in the multiplex opticalcommunication system of the third embodiment of the present invention.The optical amplifier shown in FIG. 13 consists of an optical amplifierunit (OPT-AMP UNIT) (61), an optical output monitoring unit (OUTPUT MON)(62), a gain controlling unit (GAIN CONT) (63), a constant output levelcontrol circuit (CONSTANT OUTPUT CONT) (64), an output level settingunit (LEVEL SET) (65), a control unit (CONT UNIT) (66), a control signalreceiving unit (CONT-SIG RX) (67), a gain monitoring unit (GAIN MON)(68), a constant gain control circuit (CONSTANT GAIN CONT) (69) and aselector (SEL) (70).

[0107] In case of the third embodiment, the same CONV as described inreference with FIG. 8 and the same MON CONT 7 as described in referencewith FIG. 10 are used. In FIG. 13, the same reference symbol as in FIG.9 designates the same unit or circuit as in FIG. 9, having the samefunction as in FIG. 9. Different from FIG. 9, the optical amplifiershown in FIG. 13 includes the GAIN MON 68, the CONSTANT GAIN CONT 69 andthe SEL 70. The SEL 70 is controlled by the CONT UNIT 66 so that the SEL70 usually selects the CONSTANT OUTPUT CONT 64 for controlling theOPT-AMP UNIT 61 by the GAIN CONT 63.

[0108] Upon receiving the optical output control signal from the CONTMON 7, the CONT-SIG RX 67 transfers the optical output control signal tothe next optical amplifier (for example, the REP OPT-AMP 81) and theCONT UNIT 66. When the optical output control signal signifies that theCONV is added or removed, the optical output control signal controls theSEL 70 so that the SEL 70 selects the CONSTANT GAIN CONT 69 forconnecting the CONSTANT GAIN CONT 69 with the GAIN CONT 63. As a result,control of the optical output from the OPT-AMP UNIT 61 is changed fromthe constant output level control to the constant gain control, so thatthe OPT-AMP UNIT 61 produces the optical output at an increasing rate ofthe output from the added CONV or an decreasing rate of the output fromthe removed CONV. That is, the OPT-AMP UNIT 61 produces the opticaloutput by keeping the single optical output constant individually.

[0109] When a prescribed time determined by the rising time constant ofthe added CONV or the falling time constant of the removed CONV ispassed, or when the output of the added CONV is in creased to aprescribed large value or the output of the removed CONV is decreased toa prescribed small value, the optical output control signal sends theabove information to the optical amplifier. Upon receiving theinformation, in the optical amplifier, the CONT UNIT 66 controls the SEL70 so that the SEL 70 selects the CONSTANT OUTPUT-LEVEL CONT 64 forconnecting the CONSTANT OUTPUT-LEVEL CONT 64 with the GAIN CONT 63.

[0110]FIG. 14 is a block diagram for detailing the optical amplifiershown in FIG. 13. In FIG. 14, the same reference numeral as in FIG. 13designates the same unit as in FIG. 13. In FIG. 14, symbol “ASE” in theOPT-AMP UNIT 61 is an abbreviation of “Amplified Spontaneous Emission”which will be described later, the OUTPUT MON 62 consists of an opticalbranch which will be simply called a “brancher” (73) and an opticaldetector (74), the GAIN CONT 63 consists of an optical combiner (71) anda pumping semiconductor laser (72), the GAINMON 68 includes anoptical-to-electrical signal converter (79) using e.g. a photo diode,the LEVEL SET 65 includes a switching circuit (SW) (77), the CONSTANTOUTPUT LEVEL CONT 64 includes an operational amplifier (ope-amp) (76)and the CONSTANT GAIN CONT69 consists of an averaging circuit (80), aholding circuit (81) and an ope-amp (82), and a transistor (75) is forconnecting the GAINCONT 63 and the SEL 70.

[0111] When the CONT UNIT 66 receives the optical output control signalincluding information on starting the added CONV or stopping the removedCONV through the CONT-SIG RX 67, the CONT UNIT 66 controls the SEL 70 sothat the CONSTANT OUTPUT LEVEL CONT 64 is switched to the CONSTANT GAINCONT 69. As a result, a constant gain control loop is formed byconnecting the transistor 75, the semiconductor laser 72, the opticalcombiner 71, the OPT-AMP UNIT 61, the GAIN MON 68 and the CONSTANT GAINCONT 69. By virtue of the constant gain control loop, the OPT-AMP UNIT61 can produce the optical output under the constant gain control.

[0112] In this case, the photo diode 79 in the GAIN MON 68 detects theASE emitted from the Er doped optical fiber in the OPT-AMP UNIT61. Thisdepends on a fact that the intensity of the ASE is directly proportionalto the gain of the Er doped optical fiber. Since the ASE is emitted froma side of the Er doped optical fiber, the ASE can be detected by thephoto diode 79 placed at the side of the Er doped optical fiber. (Thedetails of the above is described in Japanese Patent TOKUKAIHEI4-356984).

[0113] A detected signal from the photo diode 79 is sent to theaveraging circuit 80 and averaged thereat, producing an averaged signal.The averaged signal is sent to the holding circuit 81 and held thereatuntil the constant gain control is switched to the constant output levelcontrol. The averaged signal held at the holding circuit 81 is sent tothe ope-amp 82. The ope-amp 82 operates so that the detected signal fromthe photo diode 79 becomes always constant to the average signal sentfrom the holding circuit 8 1. This is performed by controlling theconstant gain control loop. That is, the output from the ope-amp 82controls the transistor 75 through the SEL 70 so that the pumpingsemiconductor laser 72 controls pumping optical power so as to keep thegain of the OPT-AMP UNIT 61 constant.

[0114] By virtue of the constant gain control described above, theoptical output of the OPT-AMP UNIT 61 is increased or decreased incorresponding to increase or decrease of the optical input of theOPT-AMP UNIT 61. Therefore, the OPT-AMP UNIT 61 can produce the opticaloutput so as to correspond to the increase or decrease of the outputfrom the added or removed CONV respectively. As a result, it becomespossible to add or remove the CONV by keeping the single output of theOPT-AMP UNIT 61 constant, which results in decreasing the wave shapedistortion of the optical signal and preventing the decrease of the SNRoccurring.

[0115] The CONT UNIT 66 makes the SW 77 select one of the referencevalues rf1, rf2, - - - and rfm in corresponding to the number of theoperating CONVs, and after a prescribed time passed or upon receivingthe optical output control signal from the MON CONT 7, the CONT UNIT 66controls the SEL 70 so that the connecting object of the ope-amp 76 ischanged from the ope-amp 82 to the ope-amp76. By virtue of changing theoperational amplifier thus, the pumping semiconductor laser 72 iscontrolled so that -under the constant output level control, the OPT-AMPUNIT 61 produces the optical output in accordance with the referencevalue set at the LEVEL SET 65.

[0116]FIG. 15 is a block diagram of multiplex optical communicationsystem for illustrating a fourth embodiment of the present invention. InFIG. 15, the same reference numeral as in

[0117]FIG. 3 designates the same unit as in FIG. 3. In FIG. 15,reference numeral 15A indicates a control-signal optical transmitter(CONT-SIG OPTICAL TX). The CONT-SIG OPTICAL TX 15A transmits an opticalcontrol signal including the optical output control signal and having awavelength different from the wavelengths used in the CONVs 4-1 to 4-nincase of WDM. The optical control signal is sent to the OPT-SIG COMB 5by an optical fiber depicted by a thick line in FIG. 15. The opticalcontrol signal is combined with other channel optical signals from theELEC-OPT CONV 4 and sent to the TX-UNIT OPT-AMP and the REP OPT-AMP 81through the optical fiber and the OPT-TRANS LINE 3. By virtue of usingthe optical signal, the electric line for transmitting the opticaloutput control signal becomes unnecessary to be laid.

[0118]FIG. 16 is a block diagram for illustrating an optical amplifierused in the REP OPT-AMP 81 in case of the fourth embodiment. In FIG. 16,the same reference symbol as in FIG. 9 designates the same unit and hasthe same function as in FIG. 9. In FIG. 16, the optical amplifierconsists of an OPT-AMP UNIT (91), an OUTPUT MON (92), a GAIN CONT (93),a CONSTANT OUTPUT CONT (94), a LEVELSET (95), a CONT UNIT (96), aCONT-SIG RX (97), an electro-optical converter (ELEC-OPT CONV) (98), anoptical channel demultiplexer (OPT-DEMUX) (99) and an optical combiner(OPT-COMB) (100).

[0119] The optical amplifier for the fourth embodiment shown in FIG. 16has the same constitution as the optical amplifier for the firstembodiment shown in FIG. 9 except that the optical amplifier for thefourth embodiment includes the ELEC-OPT CONV 98, the OPT-DEMUX 99 andthe OPT-COMB 100. Since the optical output control signal is sent fromthe MON CONT 7 to the optical amplifier in FIG. 16 through the OPT-TRANSLINE 3 as an optical signal, the optical signal including the opticaloutput control signal is demultiplexed at the OPT-DEMUX 99 and sent tothe CONT UNIT 97. The optical signal is converted to an electricalsignal, which is the optical output control signal, at the CONT UNIT 97.The optical output control signal is again converted to an opticalsignal for transmitting to the REP OPT-AMP 81 in the next OAMP REP EQUIP200. In the optical amplifier for the third embodiment described inreference with FIGS. 12 and 13, it is . possible to transmit the opticaloutput control signal to the REP OPT-AMP 81 as an optical signal throughthe OPT-TRANS LINE 3 and convert the optical signal including theoptical output control signal to the electrical signal at the opticalamplifier for the third embodiment.

[0120]FIGS. 17A, 17B and 17C are block diagrams for illustrating amultiplex optical communication system of a fifth embodiment of thepresent invention. Different from the multiplex optical communicationsystem of the first embodiment operating under the WDM, the multiplexoptical communication system of the fifth embodiment operates under theOTDM. In FIGS. 17A, 17B and 17C, the same reference symbol as in FIGS.2, 3 and 4 designates substantially the same unit as in FIGS. 2, 3 and4, and the same reference numeral as in FIG. 3 designates the sameobject as in FIGS. 2, 3 and 4. In FIGS. 17A, 17B and 17C, the multiplexoptical communication system principally consists of two TERM EQUIP (101and 101′), a plurality of OAMP REP EQUIP (102) and the OPT-TRANS LINE 3.The TERMEQUIP 101 includes a TX-UNIT (1111) which consists of anELECT-OPT CONV (104), an OPT-SIG COMB (105), a TX-UNIT OPT-AMP (106) anda MON CONT (107) and an RX-UNIT (1102). The ELECT-OPT CONV 104 consistsof CONVs 104-1, 104-2, - - - and 104-n, and each CONV consists of aCONV-CIRCUIT (110), an OPT-OUT MON (111) and OPT-OUT CONT (112). The MONCONT 107 consists of a MON-PROC (113), a CONT-PROC (114) and a CONT-SIGTX (115). The OAMP REP EQUIP 102 includes REP OPT-AMPs (108 and 108′).

[0121] The CONVs 104-1 to 104-n transmit optical signals in accordancewith electrical signals or data sent through the ELEC-SIG CHANNELLINES9, at the same wavelength, however, different time slot each other. TheOPT-SIG COMB 105 multiplys the optical signals sent from the CONVs 104-1to 104-n in time-divisional and produces optical output as an OTDMsignal. Control of the time slots to the CONVs 104-1 to 104-n isperformed by the CONT-PROC 114 in the MON CONT 107.

[0122] The TX-UNIT OPT-AMP 106 and the REP OPT-AMP 108 amplify the OTDMsignal under the time constant control system described in the firstembodiment and the control manner change system described in the thirdembodiment. That is, the optical amplification at the TX-UNIT OPT-AMP106 and the REP OPT-AMP 108 can be perform by making the rising orfalling time constant of the TX-UNITOPT-AMP 106 and the REP OPT-AMP 108equal to the rising or falling time constant of the added or removedCONV respectively, or the optical amplification can be performed byswitching the control manner as follows: before a CONV is added orremoved, the optical amplification is performed under the constantoutput level control as usually performed in the prior art; when a CONVis added and starts to operate, the optical amplification is performedunder the constant gain control until the optical output of the addedCONV reaches a prescribed value, and when a CONV is removed and stopsoperation, the optical amplification is performed under the constantgain control until the optical output of the removed CONV is decreasedto zero value or a prescribed value near to zero; and when the opticaloutput of the added CONV is raised to the prescribed value, the opticalamplification is performed by the constant output level control, andwhen the optical output of the removed CONV is decreased to zero valueor the prescribed value near to zero value, the optical amplification isperformed under the constant output level control. FIG. 18 isillustration of OTDM. For instance, four CONVs 104-1, 104-2, 104-3 and104-4 produce optical output at time slots T1, T2, T3 and T4 as shown by1-0 signals (a), (b), (c) and (d) in FIG. 18, respectively. Uponreceiving four optical output from the CONVs 104-1, 104-2, 104-3 and104-4, the OPT-SIG COMB 105 performs multiplex to the four opticaloutput and produces an OTDM datum as shown by (e) in FIG. 18. Controlfor making the CONVs 104-1, 104-2, 104-3 and 104-4 correspond with thetime slots T1, T2, T3 and T4 can be performed by using an ordinal methodused in usual OTDM system.

[0123] The present invention can be applied to other multiplex opticalcommunication system employing TDM-OTDM combination system forincreasing multiplex factor and operation efficiency of the multiplexoptical communication system.

What is claimed is:
 1. A system, comprising: a terminal which transmits,to an optical fiber transmission line, an optical signal including aplurality of second optical signals with different wavelengths and asupervisory optical signal, by which condition of the optical signal isnotified, with wavelength different from the second optical signals;and, an optical amplifier including: an amplifying unit which receivesthe optical signal and the supervisory optical signal from the opticalfiber transmission line and amplifies the optical signal; and, acontroller which detects the supervisory optical signal and has a firstmode in which the amplifying unit is controlled to amplify the opticalsignal with an approximately constant gain and a second mode in whichthe amplifying units is controlled to output the amplified opticalsignal with a predetermined level, the controller switchable between thefirst mode and the second mode in response to the supervisory opticalsignal.
 2. A system according to claim 1, wherein a variation of anumber of the second optical signals is notified by the supervisoryoptical signal, and the controller is switchable to the first mode inresponse to the supervisory optical signal.
 3. A system according toclaim 1, wherein steady state of a number of the second optical signalsis notified by the supervisory optical signal, and the controller isswitchable to the second mode in response to the supervisory opticalsignal.
 4. A system according to claim 1, wherein, in the first mode,the amplifying unit is controlled to output each second optical signalwith respective predetermined levels.
 5. A system, comprising: firstmeans for transmitting, to an optical fiber transmission line, anoptical signal including a plurality of second optical signals withdifferent wavelengths and a supervisory optical signal, by whichcondition of the optical signal is notified, with wavelength differentfrom the second optical signals; second means for receiving the opticalsignal and the supervisory optical signal from the optical fibertransmission line and amplifying the optical signal; and, third meansfor detecting the supervisory optical signal and for switching, inresponse to the supervisory optical signal, between a first mode inwhich the second means is controlled to amplify the optical signal withan approximately constant gain and a second mode in which the secondmeans is controlled to output the amplified optical signal with apredetermined level.
 6. A system according to claim 5, wherein avariation of a number of the second optical signals is notified by thesupervisory optical signal, and the third means is switched to the firstmode in response to the supervisory optical signal.
 7. A systemaccording to claim 4, wherein steady state of a number of the secondoptical signals is notified by the supervisory optical signal, and thethird means is switched to the second mode in response to thesupervisory optical signal.
 8. A system according to claim 5, wherein,in the first mode, the second means is controlled to output each secondoptical signal with respective predetermined levels.
 9. An opticaltransmission system, comprising: a terminal station which transmits, toan optical fiber transmission line, an optical signal including aplurality of second optical signals with different wavelengths and asupervisory optical signal, by which condition of the optical signal isnotified, with wavelength different from the second optical signals; anoptical amplifier which, optically coupled to the optical fibertransmission line, receives the optical signal and amplifies the opticalsignal; and, a controller, receiving the supervisory optical signal fromthe optical fiber transmission line, operatively coupled to the opticalamplifier and switched, in response to the supervisory optical signal,between a first mode in which the optical amplifier is controlled toamplify the optical signal with an approximately constant gain and asecond mode in which the optical amplifier is controlled to output theamplified optical signal with a predetermined level.
 10. An opticaltransmission system according to claim 9, wherein a variation of anumber of the second optical signals is notified by the supervisoryoptical signal, and the controller is switched to the first mode inresponse to the supervisory optical signal.
 11. An optical transmissionsystem according to claim 9, wherein steady state of a number of thesecond optical signals is notified by the supervisory optical signal,and the controller is switched to the second mode in response to thesupervisory optical signal.
 12. An optical transmission system accordingto claim 9, wherein, in the first mode, the optical amplifier iscontrolled to output each second optical signal with respectivepredetermined levels.
 13. A system, comprising: a terminal whichtransmits, to an optical fiber transmission line, an optical signalincluding a plurality of second optical signals with differentwavelengths and a supervisory optical signal, by which condition of theoptical signal is notified, with wavelength different from the secondoptical signals; and, an optical amplifier including: an amplifying unitwhich receives the optical signal and the supervisory optical signalfrom the optical fiber transmission line and amplifies the opticalsignal; and, a controller which detects the supervisory optical signaland has a first mode in which the amplifying unit is controlled tooutput the amplified second optical signals with respectivepredetermined levels and a second mode in which the amplifying units iscontrolled to output the amplified optical signal with a predeterminedlevel, the controller switchable between the first mode and the secondmode in response to the supervisory optical signal.
 14. A systemaccording to claim 13, wherein a variation of a number of the secondoptical signals is notified by the supervisory optical signal, and thecontroller is switchable to the first mode in response to thesupervisory optical signal.
 15. A system according to claim 13, whereinsteady state of a number of the second optical signals is notified bythe supervisory optical signal, and the controller is switchable to thesecond mode in response to the supervisory optical signal.
 16. A system,comprising: first means for transmitting, to an optical fibertransmission line, an optical signal including a plurality of secondoptical signals with different wavelengths and a supervisory opticalsignal, by which condition of the optical signal is notified, withwavelength different from the second optical signals; second means forreceiving the optical signal and the supervisory optical signal from theoptical fiber transmission line and amplifying the optical signal; and,third means for detecting the supervisory optical signal and forswitching, in response to the supervisory optical signal, between afirst mode in which the second means is controlled to output theamplified second optical signals with respective predetermined levelsand a second mode in which the second means is controlled to output theamplified optical signal with a predetermined level.
 17. A systemaccording to claim 16, wherein a variation of a number of the secondoptical signals is notified by the supervisory optical signal, and thethird means is switched to the first mode in response to the supervisoryoptical signal.
 18. A system according to claim 16, wherein steady stateof a number of the second optical signals is notified by the supervisoryoptical signal, and the third means is switched to the second mode inresponse to the supervisory optical signal.
 19. An optical transmissionsystem, comprising: a terminal station which transmits, to an opticalfiber transmission line, an optical signal including a plurality ofsecond optical signals with different wavelengths and a supervisoryoptical signal, by which condition of the optical signal is notified,with wavelength different from the second optical signals; an opticalamplifier which, optically coupled to the optical fiber transmissionline, receives the optical signal and amplifies the optical signal; and,a controller, receiving the supervisory optical signal from the opticalfiber transmission line, operatively coupled to the optical amplifierand switched, in response to the supervisory optical signal, between afirst mode in which the optical amplifier is controlled to output theamplified second optical signals with respective predetermined levelsand a second mode in which the optical amplifier is controlled to outputthe amplified optical signal with a predetermined level.
 20. An opticaltransmission system according to claim 19, wherein a variation of anumber of the second optical signals is notified by the supervisoryoptical signal, and the controller is switched to the first mode inresponse to the supervisory optical signal.
 21. An optical transmissionsystem according to claim 19, wherein steady state of a number of thesecond optical signals is notified by the supervisory optical signal,and the controller is switched to the second mode in response to thesupervisory optical signal.